Tuberculosis
The discovery of mycobacterium tuberculosis (MTB)-specific immunodominant antigens has led to a significant new avenue for the diagnosis of tuberculosis (TB). Early work had shown the potential to replace the Tuberculin Skin Test (TST) by a test that assayed the in vitro production of interferon gamma (IFN-γ) by T cells in response to defined MTB antigens. Around the same time, a major advance was the discovery of the highly immunogenic antigens, early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10) and TB7.7 that improved specificity significantly. These antigens are encoded within the region of difference 1 (RD1) of the pathogen and are consequently absent from all Bacille Calmette Guerin (BCG) vaccine strains and most non-tuberculous mycobacteria (exceptions include Mycobacterium kansasii, Mycobacterium marinum Mycobacterium szulgai). IFN-γ responses to overlapping peptides of the RD1 encoded antigens ESAT-6, CFP-10, TB7.7 form the basis for the detection of MTB infection in two licensed and commercially available tests.
QuantiFERON-TB Gold (Cellestis Limited, Carnegie, Victoria, Australia), a whole blood enzyme-linked immunoassay (ELISA) has European CE mark approval and recently received American Food and Drug Administration (FDA) approval for the detection of both latent TB infection and disease.
T-SPOT.TB (Oxford Immunotec, Oxford, UK), an enzyme-linked immunospot assay (ELISPOT) that uses peripheral blood mononuclear cells has European CE mark approval and was approved for use in Canada in 2005. T-SPOT.TB only uses ESAT-6 and CFP10.
However, the limitations of the currently available tests are:                1) The sensitivity may be impaired in immunosuppressed individuals (such as HIV positive or patients receiving immunosupressing medication),        2) In some situations a relatively large volume of blood is necessary (3 ml per QuantiFERON test and 8 ml for the T-SPOT.TB), which may limit its use in infants and severely ill and anaemic children,        3) The tests do not discriminate between active, latent and recent infection        4) The tests have not been demonstrated to be able to predict who will progress from recent or latent TB to active TB.        
Most of the test limitations are due to measurement of the effect parameter IFN-γ at very low levels, close to the limit of even the most sensitive method (in the QuantiFERON test down to 0.35 IU/ml (17.5 pg/ml) and in the T-SPOT.TB 5 spots/field). Decreasing cut-off to enhance sensitivity will eventually result in impaired specificity of the tests. A recent publication based on the QuantiFERON test has shown that that repeated testing of people with test results in the lower range of IFN-γ varies around the cut-off level which underlines the potential risk of false positive and false negative results of the QuantiFERON (QFT) test (Pai, M. et al).
To overcome the evident fragility of the tests, sensitivity could be improved by using additional M. tuberculosis specific antigens and this has been done in the third generation of the QuantiFERON tests (QFT), the QuantiFERON In tube test (QFT-IT) which now comprises an additional antigen named TB7.7(p4) and potentially sensitivity is improved, but it still depends on measurements at very low IFN-γ levels.
This approach has been tried by others, i.e. recently is was shown that the Monokine Induced by IFN-γ (MIG/CXCL9) was specifically expressed in vitro after stimulation with M. tuberculosis specific antigens (ESAT-6/CFP10) and PPD. The sensitivity of CXCL9, however was very low and lower than that of IFN-γ. Another smaller study based on intracellular cytokine cytometry in CD4+ T cells following ESAT-6 stimulation, tested if expression of IFN-γ, IL-2, IL-4, IL-10 or the activation marker CD40L could distinguish TB from non-TB disease. None of these markers were found to be comparable or superior to IFN-γ (Abramo C, et al) (Hughes, A. et al).
Yet no sensitive and specific markers to replace IFN-γ for diagnosis of TB infection have been identified in the presently published literature. Various have disclosed IP-10 in connection to infections, but not as a marker in the diagnosis of infection with a prior antigen stimulation.
Chlamydia
The diagnosis of genital chlamydial infections evolved rapidly from the 1990s. Nucleic acid amplification tests (NAAT), such as polymerase chain reaction (PCR), transcription mediated amplification (TMA), and the DNA strand displacement assay (SDA) now are the mainstays. The most commonly used and widely studied chlamydia NAATs in the US and many other industrialized countries are Aptima (Gen-Probe), Probe-Tec (Becton-Dickinson), and Amplicor (Roche). The Aptima Combo II assay tests simltaneously for C. trachomatis and Neisseria gonorrhoeae, the cause of gonorrhea. NAAT for chlamydia may be performed on swab specimens collected from the cervix (women) or urethra (men)
At present, the NAATs have regulatory approval only for testing urogenital specimens. The NAATs have largely replaced culture, the historic gold standard for chlamydia diagnosis, and the non-amplified probe tests, such as Pace II (Gen-Probe). The latter test is relatively insensitive, successfully detecting only 60-80% of infections in asymptomatic women, and occasionally giving falsely positive results. Culture remains useful in selected circumstances and is currently the only assay approved for testing non-genital specimens.
Chlamydia diagnosis is thus based on complicated and resource demanding technology such as PCR, not readily available in the developing world. A fast and easy technology would improve diagnostic measures for this important disease,
CA 2,478,138 discloses that elevated blood levels of the chemokine CXCL10 polypeptide are associated with respiratory illnesses (e.g. SARS, influenza and community-acquired pneumonia) and are useful in diagnosis of patients. Methods are provided for diagnosis and treatment of patients suffering from respiratory illnesses.
WO 05/091969 discloses markers for TSE (Transmissible Spongiform Encephalopaties) that are present prior to formation of detectable pathological prion protein are useful to detect this infection prior to clinical signs. IP-10 is just one of several markers disclose and this application does not disclose any antigen stimulation.
US2004-038201 discloses distinct gene expression programs activated in response to different pathogens in macrophages. IP-10 is again just one in many markers mentioned and this application does not disclose any antigen stimulation.
Annalisa Azzurri et al. discloses IFN-γ-inducible protein 10 and pentraxin 3 plasma levels are tools for monitoring inflammation and disease activity in Mycobacterium Tuberculosis infection. The article shows that IP-10 plasma level is spontaneously increased in patients with TB, again this reference does not disclose any antigen stimulation.
WO 03/063759 discloses a method of identifying a heat shock protein (Hsp) derived peptide useful for diagnosis or therapy. The effect of the compounds of this invention was tested on pheripheral blood monocytes by measuring the IP-10 level in response to subsequent stimulation with LPS. Direct stimulation with the test compounds without subsequent stimulation with LPS, did not give rise to an increase in the IP-10 level.
WO 07/039400 discloses a method and kit for the diagnosis of Immune Restoration Syndrome which is associated with tuberculosis (TB-IRS) in patients infected with tuberculosis as well as HIV/TB co-infected patients. In order to diagnose TB-IRS the inventors detected the level of Th1 response against PPD and/or a 16 KDA protein in comparison to the Th1 response against ESAT-6, CFP-10, 85B (negative control) and used a rise in the Th1 response as an indicative of TB-IRS.
To overcome the problems of impaired sensitivity and low levels of detectable IFN-γ by the currently available tests using antigen stimulation, the present inventors suggest the use of an alternative biomarker than IFN-γ.